The present invention relates to structured catalysts and more particularly to honeycomb structures useful as catalysts or catalyst supports for chemical reactors wherein heat management is of primary importance.
In a chemical reactor, regardless of its configuration or size, the two principal variables affecting the reaction rate are time and temperature. By controlling the heat transfer, and thus the temperature, the length of time a reaction or process required for completion can be determined. For this reason, temperature control is a critical reactor design consideration for chemical processes. Example processes wherein managing reactor heat is especially important include selective oxidations to make products such as ethylene oxide, phthalic anhydride, maleic anhydride, formaldehyde, acrylonitrile, acrolein, acrylic acid, methacrolein, methacrylic acid, methacrylonitrile, 1,2-dichloroethane, vinyl chloride, methanol synthesis, and Fischer-Tropsh synthesis.
In selective oxidation processes, it is important to manage the exothermic heat of reaction to maintain desired product selectivity. Non-selective by-products of oxidation (e.g. CO, CO2, H2O) are more thermodynamically favored than the partially oxidized intermediates, so reactor operation is a delicate balance between maximizing the production rate of desired product while avoiding over-oxidation. Temperature is the most important operating variable which affects the reaction rate, and if it is not controlled closely, selective oxidation reactions can easily run out of control, resulting in a large heat release, loss of selectivity, and possible safety and environmental hazards.
The importance of heat transfer in laboratory scale reactors for developing processes such as above described can go unnoticed because laboratory equipment generally comprises relatively small vessels having a large surface-to-volume ratio. The surface area available for heat transfer makes it simple to maintain the reaction at isothermal conditions. On an industrial scale, on the other hand, surface-to-volume ratios are much smaller and make heat transfer and temperature control much more difficult.
The use of monolithic honeycomb catalysts or catalyst supports in certain types of chemical reactors is well known. For some gas phase processes, such as the oxidation of unburned species present in the engine exhaust streams of automobiles, high reactor temperatures are permissible and honeycombs of refractory ceramic materials such as cordierite or mullite are typically employed. Heat management during the normal operation of these systems is not a major problem.
For endothermic and exothermic processes that are of interest to the petroleum refining, petrochemical and chemical industries, however, structured supports or catalysts of honeycomb configuration have not been widely used. Among other issues, the control of reactor temperature in beds of such structured catalysts or supports can be difficult.
A specific example of a field in which reactor temperature control is particularly important is in systems for the reforming of hydrocarbon feed streams to generate hydrogen-rich gases for the operation of hydrogen fuel cells. Fuel cells have become a topic of interest for applications such as motor vehicle propulsion because of their relatively high energy efficiency and low levels of pollutant emission. Most of the mobile and stationary fuel cell applications presently being considered will require fuel reforming, but while the reforming of several types of fuels has been shown to be feasible, there is an interest in reducing the size of such reforming systems, especially for transportation applications.
The reactions involved in fuel reforming are several, and may include sulfur removal from the hydrocarbon feed, partial oxidation of the feed to produce a hydrogen-rich reformate, hydrogen enrichment of the reformate through water-gas shift processing, and preferential oxidation to reduce the carbon monoxide content of the hydrogen-rich product stream. All of these reactions require good heat management, with the water-gas shift yields being particularly sensitive to temperature.
WO 00/66486 and WO 00/66487 provide examples of fuel reformer systems that could be adapted for fuel cell applications such as described. WO 00/66486 proposes a water-gas shift catalyst comprising a platinum group metal supported directly on zirconia for use in such a system, the zirconia consisting of pellets or a zirconia-containing monolith. However, a significant disadvantage of the zirconia catalysts proposed in the '486 publication is the relatively poor heat conductivity of zirconia, which presents difficulties for reactor temperature control and thus for hydrogen yield in the reactor systems described.
The use of steel honeycomb structures in catalytic reactors such as automotive catalytic converters is well known, these typically being formed by the shaping of sheet metal feedstock as disclosed, for example, in U.S. Pat. No. 5,105,539. Similar honeycomb structures of iron alloy, nickel, aluminum or copper are proposed in EP 1110605 for use as heat-conductive catalyst supports that can reduce hot spots in externally cooled tubular reactors for the selective chlorination or oxychlorination of alkenes. Iron-aluminum alloy honeycomb bodies made by the extrusion of metal powder batches through honeycomb extrusion dies, as disclosed in U.S. Pat. No. 4,758,272, are examples of honeycomb structures that could be used in this process.
For various reasons, these and other prior art extruded metal honeycombs have not yet proven satisfactory for use as catalyst substrates in many chemical reactions. Among deficiencies of the known extruded metal honeycombs are limited catalyst compatibility (due in part to excess carbon and other impurity levels), inadequate thermal conductivity for some reactions, a limited porosity range, and/or elastic properties that limit the physical durability of the substrates. On the other hand, metal honeycombs formed of aluminum or copper sheet stock demonstrate inadequate porosity, strength and durability for many catalyst support applications.